added +flex2d, tests and demos

[peter-nekrasov]

Added kernels, demos, and tests needed to solve flexural wave BVPs with the clamped, free, and supported plate boundary conditions.

• Created the +flex2d folder containing kernels used in the integral equations and representations (kern.m), stable evaluators for the flexural wave green's function and its derivatives (hkdiffgreen.m) as well as the green's function for the biharmonic equation (bhgreen.m).
• Tests for the clamped, free, and supported plate problems using the analytic solution test.
• Demos for exterior scattering problems on a starfish with all three boundary conditions.

Some additional (related) features that were added:

• arclengthder.m in @chunker, which computes the arclength derivative of some function defined on a chunker object.
• The kernel of the Hilbert transform (adjoint of S tau) was added to lap2d kernels.
• Added higher derivatives to planewave.m in the demos folder.

[askhamwhat]

looks good.